In recent years, from a perspective of preserving the environment, it is becoming popular to use a wind turbine generator utilizing wind power which is one form of renewable energy.
The wind turbine generator converts motion energy of the wind to the rotation energy of the rotor and further converts the rotation energy of the rotor to electric power by the generator. In a common wind turbine generator, the rotation speed of the rotor is about a few rotations per minute to tens of rotations per minute. Meanwhile, a rated speed of the generator is normally 1500 rpm or 1800 rpm and thus a mechanical gearbox is provided between the rotor and the generator. Specifically, the rotation speed of the rotor is increased to the rated speed of the generator by the gear box and then inputted to the generator. The term “rated” means the intended maximum operating condition and may be applied to speed, pressure, flow, displacement or power for example. The rated condition may be exceeded for short periods in use, generally not more than a few minutes and only intermittently.
In recent years, the gear box tends to become heavier and more expensive as the wind turbine generators are getting larger to improve power generation efficiency. Thus, a wind turbine generator equipped with a hydraulic transmission adopting a combination of a hydraulic pump and a hydraulic motor of a variable capacity type is getting much attention.
For instance, Patent Document 1 discloses a wind turbine generator using a hydraulic transmission including a hydraulic pump rotated by a rotor and a hydraulic motor connected to a generator. In the hydraulic transmission of this wind turbine generator, the hydraulic pump and the hydraulic motor are connected via a high pressure reservoir and a low pressure reservoir. By this, the rotation energy of the rotor is transmitted to the generator via the hydraulic transmission. Further, the hydraulic pump is constituted of a plurality of sets of pistons and cylinders, and cams which periodically reciprocate the pistons in the cylinders.
Further, Patent Document 2 describes a wind turbine generator adopting a hydraulic transmission constituted of a hydraulic pump rotated by a rotor, a hydraulic motor connected to a generator, and an operating oil path arranged between the hydraulic pump and the hydraulic motor. In the hydraulic transmission of this wind turbine generator, the hydraulic pump is constituted of a plurality of sets of pistons and cylinders, cams which periodically reciprocate the pistons in the cylinders, and high pressure valves and low pressure valves which opens and closes with the reciprocation of the pistons. By latching the piston near a top dead center, a working chamber surrounded by the cylinder and the piston is disabled, and then the displacement of the hydraulic pump is changed.
Although the hydraulic pump and the hydraulic motor are not variable displacement type, Patent Document 3 discloses a wind turbine generator having a hydraulic pump and a hydraulic motor. The wind turbine generator of Patent Document 3 maintains the rotation speed of the generator constant by adjusting the pressure of operating oil to be supplied from a hydraulic pump to a hydraulic motor. In this wind turbine generator, a discharge side of the hydraulic pump is connected to an intake side of the hydraulic motor via an inner space of the tower functioning as a high pressure tank, and an intake side of the hydraulic pump is connected to a discharge side of the hydraulic motor via a low pressure tank arranged below the tower.